1. Field of the Invention
This invention pertains to micro-sized, lightweight thermally reflective phase change material that is electrically conductive and is highly efficient at regulating temperature.
2. Description of Related Art
Phase change materials were first developed for astronauts' space gear to protect them from the extreme heat and cold experienced during space expeditions. These materials consist of paraffin waxes that are capable of storing large amounts of latent thermal energy through the process of phase changing. Conventional phase change materials offer an order-of-magnitude increase in thermal storage capacity over non-phase change based materials with the added benefit of nearly isothermal discharge. This allows the storage of large amounts of latent thermal energy without significantly changing the environment around the encapsulated phase change materials. The pigment particles are roughly 1-100 microns in diameter and are made by the Outlast company, which is currently one of the largest suppliers of such pigments. Another competing product is TEAP PCM Balls® that have an average diameter of 75,000 microns.
Thermal energy storage can be broken into two types: sensible heat and latent heat. Sensible heat is the thermal energy stored in a material as a result of an increase in temperature. Latent heat is the thermal energy that flows to or from a material without a change in temperature, but causes a phase change of the material. Materials such as water, concrete, wood, plastic and most metals, store thermal energy in the form of sensible heat. The advantage of a phase change material is the use of the latent heat that is available during the phase change process. A smaller amount of the heat storage capacity, depending on the temperature difference, consists of sensible heat. The same amount of thermal energy can be stored in a significantly smaller quantity of phase change material compared to that of water, wood, plastic, concrete, or even metal, as shown below in Table 1, wherein the encapsulated phase change materials (ECPMs) are heads and shoulders above the other materials in heat capacity:
TABLE 1i)Water63KJ/kgii)Concrete14.49KJ/kgiii)Wood27KJ/kgiv)Copper5.85KJ/kgv)EPCMs205KJ/kgHeat Capacities in KJ/kg ΔT, ΔT=15° C.=15 K=27° F.
For instance, the same amount of energy required to raise 1 kg of water to 1° C. would only raise the temperature of 1 kg of ECPMs to 0.31° C.
Conventional phase change materials are utilized in a wide array of commercial products including, but not limited to, clothing, thermal storage devices, sleeping bags, bedding, and building materials.
A few of the drawbacks to the conventional phase change materials include the fact that solar loading can be attributed to roughly ⅓ to ⅔ of the total heating of an outdoor structure. The most effective method of reducing this load is to either shade the structure or reflect as much of the thermal energy as possible. Current phase change pigments, such as Outlast's thermal additive powder, have limited thermal mass and no inherent ability to reflect any of this solar loading. They also have extremely low thermal conductivity, which creates the potential for the phase change materials to rapidly saturate thermally. Once the phase change materials have completely phase changed, thus becoming thermally saturated, they loose their isothermal properties and all of the advantages associated with phase change. Also, a large portion of apparel allows a person's body heat to escape due to fabric construction. Clothing utilizing conventional phase change materials can only regulate temperature by means of storing and releasing the thermal energy that a person's body produces. It is not very effective at reflecting body heat back to prevent its escape. It is also not effective in reducing the effects of solar heating in warm weather apparel.
Conventional phase change materials are not electrically conductive and have very low heat conductivity of about 0.18 W/m·K. A material with very low thermal conductivity will not be very effective in transferring heat from critical electronic component. Conventional phase change materials typically require coating them onto both sides of an electrically conductive aluminum foil or incorporating into electrically conductive grease to be used as an electrically conductive heat sink. This hinders their use as practical heat sinks in both commercial and military electronics.
Currently, there are five widely used processes for depositing thin conductive metal films and coatings. They are sputtering, RF and DC assisted; evaporation; chemical vapor deposition; electrochemical; and electroless deposition. All but electroless deposition have a negative effect on encapsulated phase change materials.
Sputtering, evaporation, and chemical vapor deposition require high temperatures for proper deposition of a metal film. Microencapsulated phase change materials are typically composed of a wax and an encapsulating polymer shell, both of which are highly susceptible to degradation by high temperatures. This prevents the application of a conducting layer by these methods.
Electrochemical deposition of conductive layers requires the parts being coated to be in direct contact with a cathode and an anode at all times for the reaction to occur. This works well for larger parts but it is almost impossible to simultaneously coat millions of micro particles.
The only truly feasible process to deposit a conductive layer on encapsulated phase change materials is by electroless deposition.
Particulate phase change materials can be metallized by electroless deposition with any of the following metals, or alloys of these metals: silver, gold, copper, nickel, palladium, platinum, rhodium, indium, tin, cobalt, molybdenum, ruthenium, and zinc. The aforementioned metals can be doped with various other elements from the Periodic Table. Transparent conductive oxides, such as indium oxide, tin oxide, zinc oxide, indium tin oxide, and indium zinc oxide, can also be deposited using electroless deposition.